This invention relates to an auxiliary source for a commutation circuit used in an inverter apparatus.
In static apparatus (called hereinafter an inverter) for converting DC power into AC power of a desired frequency the so-called series diode bridge type inverter includes in each arm a series connection of a controlled rectifier element (called hereinafter a thyristor) and a diode, while commutation capacitors are generally inserted between each phase on the positive side and the negative side at a point intermediate said thyristor and diode for the purpose of extinguishing the thyristors. In general commutation occurs from one phase to another with a 120.degree. conduction causing extinction of the thyristor of the subsequent phase on the positive side (or the negative side) through firing of the thyristor situated on the same polarity side. These are circuits employed with the current type inverter which is normally provided on the DC side with a relatively large DC reactor.
Extinction of a thyristor is due to the discharge of the commutation capacitor inserted between each phase, as earlier mentioned. However, at the start there may not be a sufficient electric charge accumulated on the commutation capacitor and the thyristor will not be turned off. This problem has been generally solved by providing an auxialiary source for the purpose of commutation. Such auxiliary sources are found in various forms.
The present invention relates to an improved auxiliary source circuit for commutation.